I have the strategy guide, and it clearly states that the only way to become a werewolf in Skyrim is to be in the Circle AKA Companions. Either there are people who left the Circle, or people who came from Morrowind is really far-fetched. I highly doubt there are wild werewolves though.
The only way for you to become a werewolf in the game is through the Companions. That doesn't mean there can't be wild werewolves (Sinding is one such werewolf; he isn't connected to The Circle at all, AFAIK), just that you can't be infected by them for gameplay and story reasons. In lore, there are werewolves all over Tamriel and you have a chance of catching the disease if you're scratched or bitten by one... then if you don't cure it in three days, it's "permanent". The Circle is a unique group of werewolves, not the only group.